Laser derusting is a novel method removing metal corrosion by laser. It is key technology to develop a new real-time monitoring system for evaluating the derust effect. We use a low-cost photodiode to monitor the light intensity of laser induced plasma, which transform the light signal into the voltage signal by oscilloscope. We can obtain the derusting threshold according to tendency chart of the peak voltage at different laser pulse energy. Furthermore, we can evaluate whether the metal surface has been cleaned by checking the peak voltage of repeated laser radiation at the same impact point. This rapid and effective method provides the basis for analysis and control of laser derusting technology, which has the merits of low cost and high efficiency.